1. Field of the Invention
The present invention pertains to an improved fiber-optics cable (referred to as optical-fiber cables herein) and to a corresponding fabrication process. In particular, the present invention pertains to a process of bundling optical fiber buffer tubes, to form an improved flexible optical core, and to an improved flexible optical core assembled in a SZ stranding configuration.
2. Discussion of Related Art
There are three general types of optical-fiber cable structures. Typically, for each of the three general structures, the jacket is made of a polymeric material and is extruded around what is commonly referred to as the optical core.
In the first structure, commonly known as a xe2x80x9cloose-tubexe2x80x9d construction, the optical core includes a central strength member around which buffer tubes are assembled in either a helical or SZ stranding configuration. The buffer tubes contain optical fibers or optical fiber ribbons, and the tube assembly is surrounded by a jacket. In this first structure, the tubes containing the optical fibers have relatively thick and rigid walls, which are made of a polymeric material. With this first structure, the optical fibers can be displaced relative to the tubes, in which they are housed. Cables possessing this first type of structure are described, for example, in U.S. Pat. No. 4,366,667 and European Patent EP-A-0,846,970.
In the second structure, the optical core includes a single tube, typically referred to as central tube construction, which is made of a polymeric material and which houses the optical fibers. If necessary, the central tubes include ribbons that may be assembled together in a spiral configuration. The central tube is surrounded by a jacket that is defined by a wall. Strength members can be embedded in the jacket wall.
In the third type of structure, the optical core includes buffer tubes made of polymeric material. The buffer tubes house the optical fibers and are assembled together in a helical or SZ stranding pattern. The buffer tube assembly is surrounded by a jacket, within which strength members are embedded. In this third structure, the buffer tubes are relatively thin and flexible and hold the optical fibers snugly, such that displacement of the optical fibers relative to each other and to the buffer tubes is highly constrained.
For certain installations, optical-fiber cables may be arranged such that, along certain paths, sections of the cable are vertical. In such vertical sections of optical-fiber cable, an optical core assembled in a SZ stranding configuration can unravel due to the effects of gravity, with each buffer tube tending to unwind and spread out vertically in a rectilinear manner. The risk of de-stranding associated with this undesirable phenomenon is particularly significant near the reversal points, where the winding direction of a buffer tube in a SZ stranding configuration reverses. More generally, unraveling and de-stranding can occur whenever the optical core or the optical cable is subjected to a tensile strain, e.g., during manufacturing or installation.
Optical-fiber cables with the first structure typically include a binder, which holds the buffer tubes in position, thereby avoiding the problem of de-stranding. Binders can be used with such optical-fiber cables because of the relative rigidity of the buffer tubes used in the first type of structure. This rigidity prevents any tightening stress exerted by the binder on the buffer tubes from being transmitted to the optical fibers.
However, binders cannot be used satisfactorily on optical-fiber cables with the third, flexible optical core structure because these cables use thin walled buffer tubes that offer little resistance to the crushing stresses that binders can produce. Consequently, the transverse stress exerted by a binder on the buffer tubes is easily transmitted to the optical fibers, thereby subjecting the optical fibers to stresses that can interfere with their optical performance. Thus, another technique is needed to prevent vertical sections of a flexible optical core or elongated core, assembled in a SZ stranding configuration, from de-stranding.
In addition, the use of binders is undesirable, in that they add cost and require special stranding equipment. Furthermore, accessing the cable requires cutting through the binders, leading to an additional access step. Accordingly, an alternative to the use of binders is desirable for these additional reasons.
An additional problem with known flexible optical cores, which are assembled in a SZ stranding configuration, is that it is difficult to strand the constituent buffer tubes uniformly, to form an optical core of uniform strand geometry. This difficulty stems from the flexibility of the buffer tubes used in such a core and from the absence of a central strength member to support the buffer tubes and couple them by friction. Accordingly, an improved bundling method is needed to improve the core cohesion and to maintain a certain amount of stranding (i.e., the number of turns in the S or Z direction) through subsequent manufacturing steps.
Preferably, the improved tube bundling technique would provide a buffer tube assembly that will maintain its geometry under load. In particular, ensuring a sufficient amount of stranding is essential for core cohesion and bending properties, and in order to provide predictable mid-span access. However, because flexible optical cores possess no central strength member, mechanical relaxation of the buffer tubes, as well as tension applied on the optical core (e.g. during a jacketing process step, or during routing the optical core in a splice box), may cause the buffer tubes to unravel or de-strand. As indicated above, such unraveling or de-stranding poses potential problems. Accordingly, an improved stranding method is needed to maintain ordered stranding under load.
According to one object of the present invention, it is sought to prevent vertical and/or strained sections of flexible buffer tube, fiber optic cable cores from unraveling or destranding. Additionally, a second object is to avoid the use of binders, thereby reducing costs, providing a streamlined process, and simplifying access to the cables. According to a third object of the present invention, it is sought to provide an improved stranding for optical cores formed of flexible buffer tubes, in order to promote core cohesion, to maintain a certain amount of stranding through subsequent manufacturing steps, and to maintain a uniform stranding geometry under load.
The present invention achieves these and other objectives by providing an opticalfiber cable including an assembly of buffer tubes which includes at least two flexible buffer tubes that are thermally bonded to one another. The optical-fiber cable further includes a plurality of optical fibers, which are housed within the buffer tubes. A jacket surrounds the assembly of buffer tubes.
According to a second aspect of this embodiment, the jacket is made of polyethylene and the tubes are made of polyvinyl chloride (PVC) or a thermoplastic elastomer with flexible diol segments.
According to a third aspect, the buffer tubes are contained within the jacket in either a helical or a SZ stranding configuration.
According to a fourth aspect, the optical-fiber cable includes mechanical reinforcement strands, which are preferably made of aramid and, more preferably, are positioned between the tubes and the jacket and arranged helically.
According to a fifth aspect, at least one strength member is provided at the periphery of the assembly of buffer tubes. Preferably, the strength member is embedded in a wall of the jacket.
According to a sixth aspect, the optical-fiber cable further includes water-proofing elements such as water-proofing tape that is positioned between the tubes and the jacket when in an annular assembly, expandable elements that are positioned within the jacket when in an interwoven tube assembly, and/or a filler material, which is used within the tubes.
The present invention also provides a process for the fabrication of an optical-fiber cable including forming an assembly having at least two flexible buffer tubes, the buffer tubes comprising a polymeric material and housing optical fibers. A jacket comprising a polymeric material is heated to an extrusion temperature and extruded around the buffer tubes to surround the assembly of buffer tubes. The buffer tubes are thermally bonded to one another by controlling the extrusion temperature of the polymeric jacket material.
According to another characteristic of this process, the jacket is made of polyethylene and the tubes are made of polyvinyl chloride (PVC) or of a thermoplastic elastomer possessing flexible diol segments, whereby the extrusion temperature is between 170 and 240xc2x0 C. More preferably, the extrusion temperature is between 200 and 240xc2x0 C.
According to yet another characteristic of this process, at least one filiform strength member is provided at the periphery of the assembly of buffer tubes.
A second embodiment of the present invention provides a fiber optic buffer tube formed of a low melting point material forming domains and a high melting point material forming a matrix. The domains are embedded in the matrix.
According to a second aspect of the second embodiment, the low and high melting point materials are a low and a high melting point thermoplastic, respectively.
According to a third aspect, the low melting point material constitutes less than forty percent and, more preferably, less than twenty percent of the buffer tube material.
According to a fourth aspect, the buffer tube further includes a filler. Preferably the filler includes calcium carbonate, talc, and/or super-absorbent polymer. powder. More preferably the filler constitutes less than forty percent of the buffer tube material. Still more preferably, the low melting point material constitutes less than forty percent of the buffer tube material and the high melting point material constitutes more than fifty percent of the buffer tube material.
According to a fifth aspect, the high melting point material has a melting point which is at least 25xc2x0 C. higher and, preferably, at least 50xc2x0 C. higher than the melting point of the low melting point material.
According to a sixth aspect, the low melting point material is ethylene-octene copolymer, ethylene-propylene copolymer, EVA copolymer, EAA copolymer, or some other copolymer or terpolymer of a polyolefin.
According to a seventh aspect, the high melting point material is impact modified polypropylene copolymer or propylene-ethylene copolymer. Preferably, the low melting point material is ethylene-octene copolymer, EVA copolymer, or EAA copolymer. More preferably, the buffer tube further includes a calcium carbonate filler, the high melting point material is a propylene-ethylene copolymer with a melting point above 140xc2x0 C., the low melting point material is an ethylene-octene copolymer with a melting point below 100xc2x0 C., and the domains have a length-scale of 10-100 xcexcm on an outer surface of the buffer tube.
According to yet another aspect, the high melting point material is impact modified polypropylene copolymer, and the low melting point material is very low density polyethylene (VLDPE).
The present invention also provides a method of bonding fiber optic buffer tubes. The method includes bundling a plurality of buffer tubes, which tubes are formed of low melting point domains embedded in a high melting point matrix. The method further includes thermally activating the low melting point domains, thereby softening the domains and causing a portion of the domains to bond to domains of a neighboring buffer tube.
According to another characteristic of this method, the thermal activation step is accomplished by applying a heated jacket to the plurality of bundled buffer tubes. Alternatively, the thermal activation step is accomplished by applying heat using IR radiation or a convection oven.